The present invention relates to a laminate-type electrophotographic photoreceptor to be positively charged by the aid of an organic photoconductive material.
Extensive studies are currently being made on organic photoconductive substances as a photosensitive material for electrophotographic photoreceptors (referred to hereinafter as "photoreceptors"). A photosensitive material made of an organic photoconductive substance offers many advantages in flexibility, heat stability, film-forming properties, transparency, and price over the conventional photosensitive material made of an inorganic photoconductive substance such as selenium. On the other hand, organic photoconductive substances have disadvantages in that they are poor in dark resistance and sensitivity. In actual use, the advantages are enhanced and the disadvantages are eliminated by forming the sensitive layer of the photoreceptor so as to comprise two laminated layers: one for carrier generation; and the other for the retention of surface charge and for carrier transport at the time of photoreception. Each layer is made of a properly selected material suitable for the desired function. Thus the laminated layers as a whole contribute to the improvement of electrophotographic performance.
The laminate-type photoreceptor is usually formed by laminating one over the other on a conductive substrate: a carrier generation layer containing an organic substance for carrier generation, and a carrier transport layer containing an organic substance for carrier transport. A photoreceptor of this type is used to form electrophotographic images by the Carlson process. The Carlson process consists of the steps of: charging the photoreceptor by means of corona discharge in the dark and exposing the surface of the charged photoreceptor, thereby forming an electrostatic latent image of the characters or pictures of an original; developing the electrostatic latent image with a toner; transferring the developed toner image to a support such as paper; and fixing the transferred image. After the transfer of the toner image, the photoreceptor is made ready for reuse by removing the charge and residual toner.
During operation, the photoreceptor is negatively charged. A disadvantage of this is that the surface of the photoreceptor becomes highly oxidized by a large amount of ozone generated by negative corona discharge. This makes it necessary to provide the photoreceptor or apparatus with a means to prevent the deterioration by ozone.
A counterpart of the negative charging system is the positive charging system which offers many advantages over the former. For example, it permits stable corona discharge, generates a smaller amount of ozone, and can be run with an easily manufactured developer. Unfortunately, any photoreceptor of laminate structure (comprising a conductive substrate, carrier generation layer, and carrier transport layer) suitable for the positive charging system is not yet available because there have not been found any organic carrier generation substance or organic carrier transport substance adequate for the layers of a photoreceptor of this type.
A possible way for the photoreceptor to be used with positive charging is to form a single layer from a mixture of a carrier generation substance and carrier transport substance or to form a carrier generation layer on a carrier transport layer. A disadvantage of the former is that the resulting single layer has a low capacity for carrier reception and lacks durability for repeated use. A disadvantage of the latter is that it is difficult to form the carrier generation layer thinner than 1 .mu.m, preferably thinner than 0.3 .mu.m, without deteriorating the carrier transport layer which has already been formed on the substrate. In addition, the conventional photoreceptor composed of organic layers is not as durable as the photoreceptor of selenium.
To improve the durability of the photoreceptor, there has been proposed a surface protective layer having good abrasion resistance and transparency, which is formed on the carrier generation layer. Such a surface protective layer has a disadvantage of causing streaky images at high temperatures under high humidity. To overcome this disadvantage, there has been proposed a surface protective layer of an amorphous inorganic material to be formed on the photoconductive layer of amorphous silicon, as disclosed in Japanese Patent Laid-open No. 87159/1986. According to the disclosure, the surface protective layer improves the moisture resistance and corona resistance of the photoreceptor and extends the life of the photoreceptor. Such a surface layer is characterized in that the contact angle of pure water is 40 to 70 degrees.